Guidance for noise reduction provided by tree belts
Introduction
Tree belts situated between the noise source and the receiver can reduce the noise level perceived by the receiver (Kragh, 1979). Wide tree belts which exceed 30 m could reduce the noise by 4–8 dB A (Eyring, 1946, Reethof, 1973, Cook and Haverbeke, 1974). While a wide belt of sparse trees may reduce the noise effectively, it may not be always practical in landscape design. In order to investigate the effect of noise reduction by narrow tree belts, six dense and narrow tree belts were chosen and the noise reduction effect behind them was studied. Parameters such as visibility in the tree belt (visibility), tree height, belt width, receiver and noise source height, and the distance between noise source and receiver (distance) were included in the observations. Then the above parameters were transformed into three-dimensionless parameters to interpret the effect of noise reduction. Finally, a map showing the relationships between the three-dimensionless parameters and the effectiveness of noise reduction were constructed.
Section snippets
Materials
Six kinds of tree belts which are common as hedges in Taiwan were chosen for the study. They were grown on flat areas. The ambient noise was maintained at 48±2 dB A. Each tree belt exceeded 50 m in length beyond which belt length has no effect on noise reduction according to Fang and Ling (2003). The characteristics of the six tree belts are shown in Table 1.
Experimental design
A centerline was drawn perpendicular to the length of the tree belt (Fig. 1). Two transecting lines, A and B, one on each side of and 2.5 m
Typical distribution of relative attenuation
The relative attenuation at various measuring sites of the six tree belts had a similar distribution pattern. Choosing a tree belt of Casuarina equisetifolia as an example, the lower the receiver and noise source height, the greater the noise attenuation (Fig. 2). A turning point existed at a distance approximately 40 m. The relative attenuation declined slowly with increasing distance up to the turning point, but declined rapidly beyond the turning point.
Multiple regressive model
The standardized coefficient (Beta) in
Discussion
When a noise meets a barrier, a noise shadow zone appears behind the barrier. The noise attenuation is high inside the shadow zone, but it is lower outside the shadow zone (Beranek and Vèr, 1992). Therefore, a receiver in the shadow zone can detect a noise reduction effect. The turning point (Fig. 2) found in this study should represent the edge of the noise shadow zone.
Tree height and belt width had positive relationships and visibility had a negative relationships with relative attenuation (
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